In the case where a fixed-bed flow reaction by continuously feed of a reaction raw material into a heterogeneous solid catalyst with endotherm or exotherm, a multitubular heat exchange type reactor is generally used.
Particularly, in the case of industrial execution of a vapor-phase oxidation that emits large amount of heat or a vapor-phase dehydration that absorbs large amount of heat, a multitubular heat exchange type reactor has been used. The vapor-phase oxidation that emits large amount of heat may include, for example, ethylene oxide production by oxidation of ethylene, acrolein and acrylic acid production by oxidation of propylene, methacrolein and methacrylic acid production by oxidation of isobutylene, and maleic anhydride production by oxidation of benzene. The vapor-phase dehydration that absorbs large amount of heat may include, for example, ethyleneimine production by dehydration of monoethanolamine, and N-vinyl-2-pyrrolidone production by dehydration of N-(2-hydroxyethyl)-2-pyrrolidone.
A multitubular heat exchange type reactor used industrially is equipped with several thousands to several tens of thousands of reaction tubes having an inner diameter of 20 to 50 mm and a length of 1 to 20 m and is filled with a solid catalyst, and is so designed as to remove or supply heat in a reaction by contacting a heat medium with these reaction tubes. In general, the heat medium of the multitubular heat exchange type reactor is so designed as to make the temperature uniform as much as possible in the entire region in which the reaction tubes have contact with the heat medium. Therefore, the load on the catalyst in the vicinity of the reaction tube inlet, where the reaction raw material concentration is high, is large to result in significant unevenness of deterioration of the catalyst from the reaction tube inlet to the outlet, and it may sometimes quicken the stage at which the reactor can no longer exhibit the desired performance.
For example, in the case of an exothermic reaction, heat removal from the vicinity of the reaction tube becomes insufficient, so that not only the catalyst layer temperature increases and side reactions increase, but also the catalyst may be damaged and runaway reaction may occur. In the case of an endothermic reaction, heat supply in the vicinity of the reaction tube inlet becomes insufficient, and the conversion may be reduced.
Many vapor-phase catalytic reactions carried out industrially include a step of regenerating a catalyst by periodically burning out carbonaceous materials accumulated on the catalyst. In such a case, removal of combustion heat in the vicinity of the reaction tube inlet in which a large amount of carbonaceous materials accumulated is sometimes insufficient, which may result in catalyst damages due to increase of the catalyst layer temperature. In general, it takes a long time to regenerate the catalyst since carbonaceous materials are burned gradually at a low oxygen concentration to avoid the temperature increase.
Methods known as a countermeasure against the problem caused in the catalyst layer in the vicinity of the reaction tube inlet are to reduce the reaction ratio in the vicinity of the reaction tube inlet by filling the vicinity of the reaction tube inlet with a diluted catalyst mixed with an inert material to the reaction or a catalyst with suppressed catalytic activity (Patent Document 1 and Patent Document 2). However, these methods are extremely complicated because the reaction tube inlet has to be uniformly filled with a catalyst or an inert material, and require a large quantity of labor and time. Further, these methods have a problem of increase of the pressure loss of the catalyst layer, and consequent increase of motive power for reaction gas supply.
Patent Document 3 and Patent Document 4 disclose methods for optimizing the reaction ratio in the vicinity of the reaction tube inlet by reducing the reaction tube inner diameter step-by-step from the reaction tube inlet to the outlet to control the temperature characteristics in the entire length of the reaction tubes. However, the cost of manufacturing the reaction tubes is high, and the reactor has a problem in mechanical strength at the connection part between reaction tubes having different diameters, and durability against thermal strain.
As a method for avoiding generation of hot spots, Patent Document 5 discloses a method for preventing local abnormally-high temperature part by installing a metal rod in the axial centers of reaction tubes. This method reduces the temperature gradient conventionally caused in the radius direction of reaction tubes, and is effective for suppressing temperature increase in the center parts of the tube axes. However, because the cross-sectional area of the catalyst layer gradually increases from the vicinity of the tip end of the metal rod, generation of a second local abnormally-high temperature part in the vicinity of the tip end is easily presumed.
Further, Patent Document 6 discloses a method for isolating a portion of a raw material gas from the catalyst by installing a hollow tube in the inside of reaction tubes. This method is effective for preventing generation of a local abnormally-high temperature part by flowing a raw material gas into the hollow tube and thus preventing the raw material gas from contacting the high temperature part. However, the method disclosed in Patent Document 6 divides the inside of the reaction tubes into two regions with different reaction ratios to make the catalyst load significantly uneven. For example, in Examples of Patent Document 6, two temperature peaks are observed.